1. Field of the Invention
This invention relates to a bipolar transistor, and in particular to a heterojunction type bipolar transistor with reduced offset voltage, reduced turn-on resistance, and improved efficiency as a power amplifier in portable terminals.
2. Description of Related Art
Technological advances in recent years have been attended by the growing use of heterojunction bipolar transistors (HBTs) as power transistors for portable terminals. In order to lengthen the usage time of batteries in portable terminals, it is important that the power-added efficiency of power transistors for portable terminals be improved. In the current-voltage characteristics of an HBT, there exists a region in which, when the collector-emitter voltage VCE is near 0 V, a collector current IC does not flow even if VCE is increased; by reducing this region, the HBT power-added efficiency can be greatly improved. The highest VCE at which IC does not flow is called the offset voltage, and in order to improve efficiency, it is important that the offset voltage, as well as the turn-on resistance, be lowered.
In an example of a low-offset-voltage structure, in the conventional HBT, described in Table 1 of the IEEE Transactions on Electron Devices, Vol. 50 No. 4, 2003, pp. 894-900 (hereafter “prior art example 1”), an InGaP layer with large valence band discontinuity ΔEv, as well as a GaAs spacer layer formed on top thereof, are adopted between the collector layer and the base layer. The InGaP layer acts as a hole barrier, suppressing the movement of holes between the subcollector and the base. In the structure of the conventional HBT, as shown in FIG. 17, a GaAs collector layer 9a, undoped hole barrier thin film layer 8c, GaAs spacer layer 8d, and base layer 7 are deposited in order on the subcollector layer 10.
The conventional HBT shown in FIG. 3 of U.S. Pat. No. 6,563,145 (hereafter “prior art example 2”) is configured similarly to that of prior art example 1. The cross-sectional structure of the conventional HBT disclosed in prior art example 2 is shown in FIG. 18. In this prior art example, a GaAs collector layer 9a, N-type hole barrier thin film layer 8a, N-type δ-doped (also called spike-doped) region 15, GaAs spacer layer 8d, and base layer 7 are deposited in order on the subcollector layer 10. The N-type δ-doped region 15 is provided in order to improve the connection of the conduction band between the N-type hole barrier thin film layer 8a and the GaAs spacer layer 8d. 
The conventional HBT disclosed in FIG. 1 of Japanese Unexamined Patent Application Publication No. 2002-252344 (hereafter “prior art example 3”) has a collector-up structure, but adopts essentially the same structure as that shown in prior art example 1. The cross-sectional structure of this conventional HBT is shown in FIG. 19. A structure is employed in which a GaAs collector layer 9a, N-type hole barrier thin film layer 8a, GaAs spacer layer 8d, and base layer 7 are formed in order under the InGaAs cap layer 14, which is equivalent to the subcollector layer 10.
These prior art examples essentially adopt a structure in which a hole barrier layer (8a, 8c) and GaAs spacer layer 8d are inserted between the GaAs collector layer 9a and the base layer 7.
Next, operation in a conventional HBT is explained. When, with VCE substantially 0 V, the base voltage VBE is higher than the turn-on voltage, apart from the current flowing from the base electrode 2 to the emitter electrode 1, if the current flowing from the base electrode 2 to the collector electrode 3 is large, then the collector current is negative, that is, current is flowing in the opposite direction. Here, if VCE is increased, the PN junction between base and collector changes from forward-bias to reverse-bias, the current flowing from the base electrode 2 to the collector electrode 3 is reduced, and when VCE exceeds the offset voltage, current flows in the positive direction. Hence in order to lower the offset voltage, when VCE is near 0 V, it is necessary to suppress the current flowing from the base electrode 2 to the collector electrode 3. During normal operation, it is electrons which flow from the collector electrode 3 to the emitter electrode 1 via the base layer 7, and even if the hole current is suppressed in order to reduce the current flowing from the base electrode 2 to the collector electrode 3, there is no adverse effect on the characteristic during normal operation. Hence the formation of a hole barrier, that is, a valence band discontinuity, between the base layer 7 and the subcollector layer 10 is effective for reducing the offset voltage.
A material with a wide band gap is used to form the hole barrier; but normally, in addition to the valence band discontinuity, a conduction band discontinuity also occurs, resulting in the problem that the turn-on resistance is increased. In order to mitigate the effect of the conduction band discontinuity, in the conventional HBT shown in FIG. 17, a GaAs spacer layer 8d is inserted between the undoped hole barrier thin film layer 8c for hole barrier formation and the base layer 7.
FIG. 20 shows a conduction band profile in the conventional HBT of FIG. 17. As a result of inserting the GaAs spacer layer 8d between the undoped hole barrier thin film layer 8c and the base layer 7, electrons flowing from the base layer 7 toward the collector have higher energy than the potential barrier due to the conduction band discontinuity formed in the undoped hole barrier thin film layer 8c, and so are not readily affected by the potential barrier. As a result, it is thought, introduction of the undoped hole barrier thin film layer 8c enables reduction of the turn-on resistance, and so is widely adopted.
However, there are a number of problems with the HBTs disclosed in the prior art examples 1, 2 and 3.
A first problem is the fact that by introducing the GaAs spacer layer 8d, the turn-on resistance, rather than being lowered, is raised. A second problem is that the turn-on resistance when a large current is flowing is increased. A third problem is that the base-collector capacitance is increased.